The reaction of the carbido cluster anion [Re7C(CO)21]3- with I2, in MeCN, under CO, allows the removal of the Re(CO)3+ capping moiety, affording the octahedral cluster dianion [Re6C(mu-CO)(CO)18]2- almost quantitatively. The anion, characterized by single-crystal X-ray analysis as its [PPh4]+ salt, has an idealized C(s) symmetry and consists of a carbon-centred metal octahedron surrounded by 18 terminal (three for each metal atoms) and one doubly bridging carbonyl ligand. The same species is obtained by dissolving the monoanion [Re7C(CO)22]- in MeCN. Variable-temperature C-13 NMR spectra indicate that the bridging carbonyl, in solution, is completely delocalized on the cluster edges and interchanges with the terminal carbonyls. The ion [Re6C(mu-CO)(CO)18]2- can be reversibly protonated, affording [Re6(mu-H)C(mu-CO)(CO)18]-, characterized by single-crystal X-ray analysis as its [NEt4]+ salt (CH2Cl2 clathrate). The carbonyl connectivity pattern resembles that of the parent anion, although the presence of the H ligand in a doubly bridging location (detected by the atom-pair potential method) lowers the symmetry to C1. The ion [ReC(mu-CO)(CO)18]2- is also able to react with Group 11 cations, as Ag+, Cu+ or Au(PPh3)+, giving adducts which can be considered intermediate between molecular complexes and ion pairs, on the basis of the nu(CO) frequencies and of the C-13 NMR spectrum of the silver derivative.
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